Question

A child is moving on a skateboard with a speed of $2m{s}^{-1}$. After a force acts on him, his speed becomes $3m{s}^{-1}$. Calculate the work done by the force, if the mass of the child is $20kg$.

• A. 1 J
• B. 10 J
• C. 20 J
• D. 320 J

Answer 1

The correct option is B

10 J

Given: initial velocity of the child, $u=2m{s}^{-1}$, final velocity, $v=3m{s}^{-1}$, and mass, $m=20kg$.

Initial kinetic energy $=\frac{m{u}^2}{2}$
Final kinetic energy $=\frac{m{v}^2}{2}$

Change in kinetic energy $=\frac{m{v}^2}{2}–\frac{m{u}^2}{2}$
$\Rightarrow \frac{m\times (v^2-u^2)}{2}=\frac{20\times (9–4)}{2}=50J$

Work done by force = change in kinetic energy of the child

Therefore work done by the force is $50J$